The overall aim of this proposal is to compare the safety and the accuracy of Chorionic Villi Sampling (CVS) with amniocentesis by means of a multicenter, cooperative clinical study. The specific hypotheses to be tested are whether the genetic analyses performed on chorionic villi are as accurate as those performed on amniotic fluid cells, particularly in the identification of chromosomally abnormal conceptions; and, whether CVS is associated with spontaneous abortion and other complications of pregnancy such as infection, low birth weight, congenital malformations or other problems in the perinatal period, and any other problems identified at later follow-up. The clinical study will provide for the randomized comparision of 2500 women undergoing prenatal genetic diagnosis and will consist of four sequential phases: 1) the development of a protocol and operations manual and training of personnel (six months duration); 2) initiation of the project by 2-4 clinical centers (six months); 3) the recruitment of 2-4 additional clinical centers, completion of the enrollment of patients (28 months), and follow-up of enrolled patients (8 months); and, 4) the analysis and reporting of data (12 months). As a participating clinical center, Michael Reese Medical Center documents its extensive experiences in prenatal genetic diagnoses, specifically with CVS as well as with amniocentesis, and its willingness to cooperate in a multicenter study; documents the availability of up to 25 patients per month who are likely to accept randomization for genetic diagnoses in the first (CVS) and second (amniocentesis) trimesters of pregnancy and presents a detailed plan for recruitment, randomization and conduct of a comprehensive multicenter study. Michael Reese Medical Center will remain available to provide prenatal sevices for women who do not wish to be randomized.